Structures such as buildings and bridges have traditionally been designed to support their own weight plus that of expected loads from people, vehicles, furnishings, etc. Buildings and other structures for supporting weight have long been expected to be very strong under vertical compression. Concrete is a favorite material for weight-bearing structures because it is inexpensive and has exceptional compressive strength.
However, many existing commercial and public structures are not strong enough to survive having one or more support columns destroyed by an explosion, earthquake, or impact. These existing structures need to be reinforced in order to meet current standards of safety. The related applications listed in the Cross-Reference section, above, disclose various methods for reinforcing the attachment among various components of a structure, such as beams, decks, walls, and columns in order to increase the structure's strength and safety.
In some cases, reinforcement of support columns themselves, in addition to connection of components, is necessary to provide sufficient safety. In other cases, reinforcement of support columns alone is sufficient to make the structure safe.
Conventional methods of reinforcing support columns can be broadly described as adding one or more additional layers to the column: pouring additional concrete around the column; welding metal supports, such as panels or bands, around the column; or wrapping the column in fiber-reinforced plastic sheathing. The purpose of adding more material is to allow the column to sway and deform, such as in an earthquake or hurricane, without the internal steel rods or bars buckling and possibly rupturing the column. The columns of many existing structures were designed without sufficient constraint of the internal steel.
Fiber-reinforced plastic (FRP) wrapping is a preferred method because it can be installed quickly with little disruption to the use of the structure. FRP material can be viewed as either a fabric that is saturated with polymer resin, or plastic that includes embedded fabric. The fabric is typically woven or knitted from fibers with high tensile strength, such as graphite carbon or high-strength glass.
FRP may be applied to a column while the resin is “wet”, i.e., not yet cross-linked and containing solvents, or when the resin is gelled and has little solvent, but not yet cross-linked. FRP may also be created in situ by wrapping the column with fabric then saturating the fabric by applying resin with a roller, sprayer, or brush.
The FRP sheathing has low mass, so it can be installed on upper floors of a building without increasing the load on lower floors. FRP sheathing is relatively thin and can conform to the original contours of the building. FRP sheathing increases the apparent ductility of the column so that it is more resistant to forces other than vertical compression. Also, if the reinforced column does fail under catastrophic forces, the failure will typically be more gradual than that of a column reinforced with concrete or metal, allowing occupants time to escape the building or even time for emergency repairs to be performed.
FRP sheathing has been widely accepted as an effective method of reinforcing standard columns of rectangular and cylindrical cross-section. However, some existing buildings have columns of more complex shape in cross-section, including concavities or re-entrant corners. Conventional FRP sheathing is considered less effective for these types of columns because of the potential for adhesive failure on complex surfaces. However, steel or concrete jacketing are undesirable because they destroy the aesthetic effect of the shaped columns. As the state of Washington Dept. of Transportation says about one of their bridges, “The bridge has cruciform “+” shaped columns that make it architecturally unique as well as a challenge to strengthen against earthquakes using steel column jackets.”
There is thus a need for a method of reinforcing support columns of complex shape that will preserve the many benefits and advantages of FRP sheathing, including retention of historic or aesthetic features, while overcoming the potential shortcoming of possible adhesive failure.